The present invention relates to a control apparatus which releases and connects under pressure the steering clutch of a tractor. The controlling apparatus of steering clutches of this kind has already been described in, for example, U.S. Pat. No. 3,262,525. The apparatus involved in the present invention is a liquid pressure apparatus which transmits the clutch releasing force to either one of the two steering clutches. The clutches are installed on the driving units of the crawlers located on both sides of the tractor and when a clutch on one side is released, the crawler on one side only is driven to preform the steering action in a well known manner. Another example is given in, for instance, Japanese Pat. No. 46-23845; this corresponds to the U.S. patent application Ser. No. 626,491.